Multi-Beam Semiconductor Laser Device
In recent years, there is a demand for the increase in the operation speed and the capacity of an optical disc device, a laser beam printer, a copying machine and the like device making use of a semiconductor laser as a light source. To meet the demand for the increase in the operation speed and the capacity, it has been proposed to utilize a multi-beam semiconductor laser capable of emitting a plurality of laser beams referred to as multiple beams in the following explanation) as a light source. For example, in the case of an optical disc device, it is possible to increase the read rate thereof by reading data from a plurality of tracks with a plurality of laser beams at the same time by the use of a multi-beam semiconductor laser.
On the other hand, while the optical output level required of the light source of an optical disc device is only several tens of mW at most, a semiconductor laser device can be used in the field of laser processing technologies and in the field of medicine by increasing the optical output level thereof to the order of W. Accordingly, the multi-beam semiconductor laser study is in works to increase the total optical output level of laser light.
Next, with reference to FIG. 10 and FIG. 11, the configuration of a prior art multi-beam semiconductor laser device is explained.
The multi-beam semiconductor laser device 10 as illustrated in FIG. 10 is capable of emitting two laser beams, and composed of two laser oscillating regions 14A and 14B sharing a common substrate 12, and provided with electrodes 16A and 16B respectively on the laser oscillating regions 14A and 14B. An opposite electrode 17 located opposed to the electrodes 16A and 16B is provided on the rear surface of the substrate 12 as a common electrode.
Furthermore, the multi-beam semiconductor laser device 10 is provided with two contact electrodes 18A and 18B to be in contact with the electrodes 16A and 16B respectively, and a base substrate 22 provided with wiring patterns 20A and 20B with which the contact electrodes 18A and 18B are connected to external terminals, wherein the electrodes 16A and 16B are electrically and mechanically connected to the contact electrodes 18A and 18B in the form of an integrated multi-beam semiconductor laser device configuration.
The multi-beam semiconductor laser device 24 as illustrated in FIG. 11 is capable of emitting four laser beams, and composed of four laser oscillating regions 28A, 28B, 28C and 28D sharing a common substrate 26, and provided with electrodes 30A, 30B, 30C and 30D respectively on the laser oscillating regions 28A to 28D. An opposite electrode 31 located opposed to the electrodes 30A to 30D is provided on the rear surface of the substrate 26 as a common electrode. Furthermore, the multi-beam semiconductor laser device 24 is provided with four contact electrodes 32A, 32B, 32C and 32D to be in contact with the electrodes 30A to 30D respectively, and a base substrate 36 provided with wiring patterns 34A, 34B, 34C and 34D with which the contact electrodes 32A to 32D are connected to external terminals, wherein the electrodes 30A to 30D are electrically and mechanically connected to the contact electrodes 32A to 32D in the form of an integrated multi-beam semiconductor laser device configuration.
GaN Base Semiconductor Laser Device Fabricated by Using the ELO Technique
On the other hand, in the field of optical memories, there is a need for a semiconductor laser device capable of emitting short-wavelength light in order to increase the recording density of an optical recording medium such as an optical disc. To meet it, research is actively carried out into gallium nitride (GaN) III–V group compound semiconductors (referred to as GaN base compound semiconductors in the following description.
A GaN base semiconductor laser device is composed generally of a laminate structure grown on a substrate. Since the device characteristics of a GaN base compound semiconductor device are affected by the crystalline condition of a GaN base compound semiconductor layer grown on a substrate, it is required for the purpose of good device characteristics to form the laminate structure of the GaN base compound semiconductor in which a few crystal defects exist.
However, since no appropriate substrate has been found which can provide a good lattice match with a GaN semiconductor as a base substrate on which a GaN base compound semiconductor layer is grown, a sapphire (α-Al2O3) substrate has been used for this purpose while the lattice constants of a sapphire substrate and a GaN layer are different from each other resulting in lattice mismatch, and in addition to this, the coefficients of thermal expansion are largely different from each other.
When a substrate provides only a poor lattice match with a GaN base compound semiconductor layer, there is generated a substantial strain within the GaN base compound semiconductor layer grown on the substrate, and therefore the crystallinity suffers from various adverse effects. For example, in order to lessen the strain as generated, a large amount of dislocations are generated in the GaN base compound semiconductor layer on the sapphire substrate with a density of the order of 108 cm−2 to 1010 cm−2.
The dislocations include threading dislocations which propagate in the thickness direction of the GaN base compound semiconductor layer, reach an active layer formed in the GaN base compound semiconductor layer, and become detrimental crystal defects which function as current leakage paths, non-light-emitting centers, and so forth to deteriorate the electric and optical characteristics of the device. Accordingly, the generation of threading dislocations has to be minimized in order to fabricate a GaN base semiconductor device having good device characteristics. From this view point, in recent years, ELO (Epitaxial Lateral Overgrowth) techniques are developed as an effective technique to lessen the generation of threading dislocations by the use of epitaxial growth in the lateral direction.
The ELO techniques are generally classified into two types, i.e., mask-using ELO techniques and free-standing ELO techniques (referred to as FS-ELO techniques in the following description).
In accordance with an FS-ELO technique, after growing a GaN base layer on a sapphire substrate, a concavo-convex stripe pattern is formed on the GaN base layer by etching the GaN base layer, for example, by reactive ion etching (RIE). The concavo-convex stripe pattern is formed as a concavo-convex structure in the form of stripes including concave areas formed by removing the GaN base layer and the most upper surface portion of the substrate to expose the substrate, and convex portion formed by the GaN base layer on the upper surface of the substrate. Next, in accordance with the technique, a GaN layer is epitaxial grown on the concavo-convex structure in the upper direction while the epitaxial grown GaN layer is further grown in the lateral direction to fill the concave areas. The dislocation density has been evaluated to be low in an area formed by epitaxial growth in the lateral direction (referred to as a wing area in the following explanation).
In the following description, the growth in the lateral direction by the FS-ELO technique will be explained together with the shortcomings thereof in further detail with reference to FIG. 16A to FIG. 16C and FIG. 17A and FIG. 17B.
First, as illustrated in FIG. 16A, a GaN layer 174 is formed on a sapphire substrate 172. Since there are lattice mismatch and thermal mismatch between the sapphire substrate 172 and the GaN layer 174, a high density defective region 176 is formed in the GaN layer 174 adjacent to the substrate as shown in FIG. 16B.
Meanwhile, in the case where a buffer layer of GaN, AlN or the like is formed on the sapphire substrate 172 in advance of forming the GaN layer 174, the high density defective region 176 of the buffer layer is formed in the vicinity of the sapphire substrate 172.
More specifically speaking, the seed defects as generated in the high density defective region 176 include stacking defects, dislocation loops having components extending approximately in parallel to the crystal growth plane, and threading dislocations extending approximately in parallel to the growth direction. Among them, the threading dislocations extending approximately in parallel to the growth direction furthermore extend from the high density defective region 176 into the GaN layer 174.
Next, after forming a mask in the form of stripes (not shown in the figure) on the GaN layer 174, a concavo-convex structure is formed on the surface of the substrate as illustrated in FIG. 16C by etching the GaN layer 174 and the upper portion of the sapphire substrate 172. In the following explanation, the convex portion is referred to as a seed crystal area 178.
Next, after removing the mask formed on the seed crystal areas 178 by chemical etching or the like, a second GaN layer 180 is grown on the upper portion of the seed crystal areas 178, while growing between the seed crystal areas 178 as the wing areas 182 by epitaxially growing the second GaN layer 180 under such a growth condition that it is grown mainly in the lateral direction as illustrated in FIG. 17A When the wing areas 182 are formed by the second GaN layer 180, a vacant space 184 is formed between the second GaN layer 180 and the sapphire substrate 172.
In this case, crystal defects are generated within the wing areas 182 during the growth in the lateral direction when the second GaN layer 180 is formed. Namely, as illustrated in FIG. 17B, dislocations 186A and 186B are formed from the high density defective region 176 in order to propagate approximately in parallel to the substrate while, among these dislocations, the dislocations 186A are bended at a meeting location 188 and extended in the vertical direction therefrom. Also, the dislocation 186B is bended in the vicinity of the meeting location 188 and extended in the vertical direction. Furthermore, threading dislocations 190 are observed which are extended through the seed crystal area 178 in the thickness direction of the second GaN layer 180 from the high density defective region 176.
As explained in the above, while high density defective region are generated above the seed crystal areas 178 and the intermediate location between adjacent seed crystal areas 178, low density defective regions are regions of the wing areas 182 as located between the meeting location 188 and the respective seed crystal area 178. Accordingly, the high density defective regions and the low density defective regions are periodically located corresponding to the periodical arrangement of the seed crystal areas.
First Object of the Present Invention
On the other hand, there are the following problems of the multi-beam semiconductor laser device in accordance with the conventional technique as illustrated in FIG. 10 and FIG. 11.
The first problem is that it is difficult to apply the configuration of the prior art multi-beam semiconductor laser device to a semiconductor laser device, such as a GaN base semiconductor laser device, which is provided with a p-type electrode and an n-type electrode in the same side of the substrate.
The second problem is that, in the case of a GaN base semiconductor laser device, which is provided with a p-type electrode and an n-type electrode in the same side of the substrate, it is difficult to fabricate a multi-beam semiconductor laser device capable of emitting respective laser beams with uniform optical output levels.
The third problem is that since each electrode on the laser oscillating region is connected to the corresponding contact electrode, it is extremely difficult to align these electrodes with each other in the case where the interval between the respective laser oscillating regions becomes narrow.
In other words, since the interval between the respective laser oscillating regions and the interval between the respective contact areas are small, one contact electrode can be connected to two laser oscillating regions only when the location of the contact electrode is slightly displaced with respect to the electrodes of the laser oscillating regions. As a result, the device yield of the multi-beam semiconductor laser device will drop. Accordingly, it is difficult in practice to increase the optical output level by increasing the number of laser beams.
It is therefore the first object of the present invention to provide a multi-beam semiconductor laser device in which the optical output levels of the respective beams are equal to each other while alignment is easy.
Second Object of the Present Invention
While the multi-beam semiconductor laser study is in works to increase the total optical output level of laser light as described above, the laser characteristics tend to be degraded while the device life cycle is shortened if a laser oscillating region is arranged on a high density defective region where crystal defects are generated due to dislocations when a multi-beam semiconductor laser is fabricated by the ELO method.
However, unless the location of a laser oscillating region has been determinate with respect to high density defective regions and low density defective regions, the attempt to locate a laser oscillating region above a low density defective region ends up in failure. From this fact, it is difficult to fabricate a multi-beam semiconductor laser device having good laser characteristics, a high optical output level and a long device life cycle.
Accordingly, the second object of the present invention is to provide a multi-beam semiconductor laser device having good laser characteristics and a high optical output level.